1. Field of the Invention
The present invention relates to the field of semiconductor processing. More particularly, the invention relates to a plasma chamber having a diagnostic head assembly for detecting the completion of a plasma etch process, monitoring the plasma characteristics in a plasma reaction and determining the build up of material on the chamber walls using both optical and/or non-optical techniques.
2. Background of the Related Art
Sub-half micron feature size and multilevel integration are two of the key technologies for the next generations of very large scale integration ("VLSI") and ultra large scale integration ("ULSI"). Plasma processing and plasma-assisted processing are essential to the production of present and future generations of large scale integrated circuits. The advantages of plasma processing are being applied to thin-film processes that involve a deposition or removal of metals, semiconductors, inorganic insulators, and organic films. Increased production rates, more precise production control and the unique material properties that can result from the non-thermal chemistry of plasma processing are significant compared to conventional processing techniques. Because of the complexity of the physical and chemical environment in a process plasma, a large array of process monitors, historically termed "plasma diagnostics" are required to characterize the plasma, or to properly monitor important control parameters. The multilevel interconnections that lie at the heart of this technology require very precise processing to fabricate high quality, uniform gates, metal wires, plugs and other features. The uniformity and performance of these features is effected by numerous factors, including the methods and apparatus used to control plasma process conditions.
As the density of semiconductor devices increases, plasma etch processes are increasingly utilized because such processes can be employed to etch films in situ and avoid wet etch techniques. A typical plasma etch system includes a plasma processing chamber and a workpiece holder to support the workpiece in the chamber. Inlet ports introduce a reactant gas or gases into the chamber where electrodes are used to excite the gases into a plasma state in the chamber. One or more of the electrodes may be excited by a direct current (DC) voltage source or a radio frequency (RF) voltage source, often at frequencies ranging between about 2 MH.sub.z and about 13.56 MH.sub.z to couple energy from the power supply into the plasma. Typical methods for transferring the power into the gas include direct coupling, capacitive coupling, and inductive coupling. Often, coupling consists of a combination of these methods, even when it is not intentional. In addition, inductors (or coil arrangements) can be used in a chamber arrangement to inductively couple power into the process chamber to excite gases introduced in the chamber into a plasma state.
Plasma etch processes can be used to etch metals, semiconductors, inorganic insulators and organic films using reactive gases. Typical reactive gases include fluorine-containing gases, such as NF.sub.3, SF.sub.6, CHF.sub.3, CF.sub.4 or C.sub.2 F.sub.6, sometimes in combination with O.sub.2, Ar, N.sub.2, or He. These gases are useful in obtaining desired etch rates, selectivities and uniformities, all of which must be precisely controlled.
A typical process for etching a contact via is described in Arleo et al, U.S. Pat. No. 5,176,790, which discusses etching a dielectric in a plasma using a mixture of fluorine-containing gases and nitrogen-containing gases. Dry etching of metals, such as aluminum, can be performed either by reactive ion etching (RIE) or by plasma etching in the presence of a halide gas, such as chlorine or bromine-containing gases for aluminum etching and fluorine-containing gases for tungsten etching. In addition, dry etching of a laminated film consisting of a metal silicide layer and a polycrystalline silicon layer can be performed using a mixed gas of SF.sub.6 and O.sub.2. Other gas combinations such as HBr, Cl.sub.2 and O.sub.2 ; NF.sub.3, Cl.sub.2 and O.sub.2 ; Cl.sub.2 and O.sub.2 ; as well as other gases can also be used to perform dry etching processes.
The increased demands placed on dry plasma etching make it extremely important to achieve reproducibility of etch results from wafer-to-wafer and batch-to-batch. In addition, the required feature characteristics need to be achieved at high wafer throughput requiring higher etch rates while achieving precise selectivity and uniformity across the wafer surface. Finally, the etching process and accompanying plasma chemistry should have a minimum effect on the etching system and the maintenance of that system.
In order to obtain a substantially uniform etch across the substrate surface, it is necessary to create and maintain a uniform plasma over the substrate and to monitor changes in the plasma. Typically, optical sensors are located within a single port on the chamber to detect changes in plasma characteristics. The presence of sensors and probes in the plasma region of the chamber, however, may disturb the plasma uniformity. Consequently, the use of sensors or probes is generally limited to a single sensor positioned in a small defined area along the perimeter of the chamber.
In applications where it is most important to determine a plasma's internal discharge parameters, an electrostatic probe, such as a Langmuir probe, may be used to measure the plasma density, charged particle concentration, and energy distribution functions. The Langmuir probe is typically a metallic electrode of cylindrical, planar, or spherical geometry, which collects current from a plasma when a voltage is applied to the probe. The probe's current collection properties, often called the probes's "current-voltage (I-V) characteristic" or the "probe characteristic", yields information on the plasma's internal discharge parameters. The probe's current-voltage characteristic is very useful for studying plasma parameters in a wide variety of situations. Langmuir probes suitable for use in RF plasmas are described by Carlile in U.S. Pat. No. 5,339,039, entitled "Langruir Probe System For Radio Frequency Excited Plasma Processing System," which is incorporated herein by reference.
In other applications where it is most important to closely monitor the etch rate of thin films on a substrate and material build up on the chamber walls, it can be beneficial to use a quartz crystal microbalance (QCM) to monitor the amount of material etched from the substrate and deposited on the chamber walls. Piezoelectric quartz crystals are commonly used to monitor the quantity of film material deposited on the crystal. These basic techniques are described in U.S. Pat. Nos. 4,817,430, 4,207,836, and 4,311,725, which are incorporated herein by reference. A circuit for monitoring a piezoelectric crystal for changes in its resonant frequency as a film is deposited on the crystal is described in U.S. Pat. No. 5,117,192, which is incorporated herein by reference. One such QCM which can be used to advantage with the present invention is the XTM/2 Deposition Monitor available from Leybold Inficon Inc. of East Syracuse, N.Y. However, other QCM's are available from other vendors and may be used to advantage in accordance with the present invention.
In plasma etch processes where it is most important to know when an etch process is complete, an endpoint detector is typically used to sense changes in the gas composition within the chamber. One such endpoint detection device is available from Applied Materials, Inc., Santa Clara, Calif., and is known as the Endpoint Optical Emission System. This system allows the user to define the etch endpoint algorithms for each etch chamber, store the endpoint data for each etched wafer and to play back the endpoint trace. The Endpoint Optical Emission System uses spectral emissions of the plasma to sense endpoint.
However, despite the availability of these instruments to monitor and control individual aspects of plasma processing, there remains a need to increase the degree of control over the entire plasma process and plasma chamber and to precisely determine the intervals between which clean processes must be performed in the chamber. In particular, there is a need for an apparatus that will provide improved monitoring and control over plasma processes, particularly plasma etch processes. It would be desirable if the apparatus could indicate the etch rate, accurately determine the endpoint of the etch process and monitor material build up on the chamber walls. Furthermore, it would be desirable if the apparatus could use the existing detection port found in typical plasma chambers and maintain the uniform plasma characteristics within the chamber. It would also be desirable if the apparatus could improve reproducibility of process results from wafer-to-wafer, batch-to-batch, achieve greater wafer throughput, have a minimum effect on the plasma processes and have a minimum effect on the maintenance of that system.